1. Field of the Invention
The present invention relates generally to the field of ballast circuits for fluorescent lamps. The invention specified relates particularly to an electronic fluorescent lamp ballast circuit using a capacitive element providing full range dimming while using a minimum number of components for reliability and having capability for operating at a high leading power factor.
2. Description of the Prior Art
Fluorescent lamps have a negative resistance characteristic once the gas in the lamp is ionized. This means that as its current begins to increase through the lamp, the resistance of the lamp decreases. This resistance decrease causes the current to further increase so that, unless some current-limiting ballast is provided, the lamp will be destroyed.
Thus, a ballast system is required which will enable the lamp to operate at a sufficiently high current for proper illumination, but will prevent the current from increasing to a level at which the lamp will destroy itself. In addition, a fluorescent lamp exhibits a very high effective resistance until the gas within the lamp ionizes, at which time a much lower resistance is presented. For that reason, the fluorescent lamp requires a high starting voltage in order that the lamp may be ignited.
For many years, iron-core transformer ballast systems have been utilized. Such designs were the only economical type available which were capable of providing a high starting voltage and, at the same time, being capable of limiting the operating current to an appropriate level. Such iron-core ballast circuits were used extensively despite several undesirable characteristics, including low power efficiency, an audible buzz and high weight.
There have been a number of attempts to improve the power efficiency of fluorescent lamp ballast systems in general, and such attempts have led to the provision of solid-state high-frequency electronic ballast systems. High frequency is desired because both the ballast system and the fluorescent lamp themselves are more efficient at frequencies at or above 400 Hz. Relatively recently small high-frequency solid-state ballasts have become available which are capable of being operated in conjunction with individual fluorescent lamp fixtures. These more recent solid-state ballast systems have the advantage over the prior-art iron-core ballast in that they are smaller size, lower weight, have virtually no audible noise and increased power efficiency.
Problems encountered with prior-art solid-state ballast systems are that, after the lamp has reached its ionization state, it exhibits negative resistance characteristics. This means that its resistance varies inversely with applied power or current. This negative characteristic is normally more easily controlled by iron-core transformers than by solid-state circuitry. This is because most of the appropriate solid-state circuits are constant voltage output devices which cannot accommodate the extreme reduction in the effective resistance of the fluorescent lamp when its gas ionizes.
The solid-state ballast system of the present invention, however, as will be described, overcomes these problems by providing a system utilizing a power circuit and a control circuit. The power circuit includes a gapped power transformer, which is connected between a 400 Hz source of electric power and the fluorescent lamp. The control circuit includes a power field effect transistor connected in an electrical circuit relationship with a ballast capacitor. Control voltages are applied to the input terminals of the control circuit for providing the dimming and brightening control signal for controlling the brightness of the fluorescent lamp or lamps, which are part of the circuit.
A second major problem encountered in the use of solid-state ballast system in the prior art, and one that has not been adequately solved prior to the present invention, is that of power factor. Power factor is the ratio of real power to reactive volt amperes. A high or leading power factor in excess of 75% is generally desirable in fluorescent light application for reasonable power efficiency. The present invention provides a leading power factor while at the same time offering stable light output throughout the dimming range.